Previous work has identified a mechanism for the transport of simple, monofunctional weak electrolytes in rat small intestine. It is suggested that this mechanism may play a significant role in the transport of a variety of compounds of physiological, pharmacological, or toxicological importance in the intestine, and at other epithelia throughout the body. A broad study is proposed in which is investigated the transport of biologically significant weak electrolytes across epithelia. The compounds to be studied will include metabolites, such as bilirubin; narcotic amines, narcotic agonists, and other drugs; some water soluble vitamins; and compounds used in the clinical and radiological examinations of hepatic and renal functions. Attention will be directed chiefly to the transport of such compounds at the epithelia of the alimentary tract, the gall bladder, and a model of the renal tubule (amphibian urinary bladder), but the proposed studies may also include observations on the transport of weak electrolytes in the liver, pancreas, kidney, choroid plexus, and the epithelia of the eye.